Stereoselective Ring-Opening Metathesis Polymerization with Tungsten Sulﬁdo Alkylidene N -Heterocyclic Carbene Complexes

A series of cationic tungsten sulﬁdo alkylidene N -heterocyclic carbene (NHC) complexes (W01 – W09) of the general formula [W(S)(CHCMe 3 )(X)(NHC) (CMe 3 CN) + B(Ar F ) 4 − ] (NHC = 1,3-dimesitylimidazol-2-ylidene, IMes; 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene, IMesCl 2 ; 1,3-bis(2,6-xdiisopropyl)phenyl)imidazol-2-ylidene, IDipp; X = Cl, C 6 F 5 O, 2,6-Ph 2 -C 6 H 3 ; B(Ar F ) 4 − = tetrakis(3,5-bis(triﬂuoromethyl)phenyl)borate) are used as initiators in the stereoselective ring-opening metathesis polymerization (ROMP) of ( + ) 2,3-endo, exo -dicarbomethoxynorborn-5-ene (( + )DCMNBE, M1). Trans -isospeciﬁty up to 84% is achieved along with varying percentages of cis -syndiospeciﬁty. The diﬀerent extent of trans -isospeciﬁty is compared to the one of related benchmark cationic molybdenum and tungsten imido and tungsten oxo alkylidene NHC complexes. Mechanistic investigations suggest that the syn -isomer of a nitrile-free initiator reacts with M1 presumably in an ene anti fashion to yield a syn -ﬁrst insertion product via turnstile rearrangement, which accounts for the predominant trans -isospeciﬁty of the polymerization.


Results and Discussion
In order to allow for a direct comparison with the stereoselectivity of other W alkylidene-based initiators, monomer M1 was chosen.Its ROMP was accomplished using initiators W01 -W10 (Figure 1).The stereochemistry of the resulting chiral polymers was determined on the basis of the 1 H NMR spectra.Cis-st and trans-st polymers possess a C 2 -axis in plane of and perpendicular to the double bond, respectively (Figure 2).Consequently, the two olefinic protons located at one double bond are not coupled to each other, yet form doublets in the 1 H NMR as a result of the coupling to the neighboring methine protons.The resulting, small 3 J-coupling constants typically give raise to pseudo-triplets.Cis-it and trans it polymers do not possess any element of symmetry.Consequently, the olefinic protons are coupled and a doublet of doublet (dd) is formed.
The -donor capabilities of the NHCs used here, reflected by the Tolman electronic parameter (TEP [33] ), were rather similar and can therefore not be expected to be decisive for the stability of the tungstacyclobutane and thus on its propensity to undergo the turnstile rearrangement (vide infra).Thus, complexes W03, W06, and W09 based on the NHC with the highest TEP (IMesCl 2 , TEP = 2054 cm −1 ) allowed for similar trans-isospecifities than all other complexes bearing the NHC with lower TEP values (Dipp, Table 1.Conversion and selectivity in the polymerization of M1 with initiators W01 -W10 as well as number-average molecular weights (M n ) and Ð of the polymers prepared.

Cat. conversion [%] trans-it
TEP [44]  TEP = 2050.5cm −1 ; IMes, TEP = 2050.5cm −1 ).Also, in view of the fact that the NHC is trans to the tungstacyclobutane in the transition state, no significant steric effects of the NHC can be expected, the more since the values for the volume buried by the individual NHCs in W04, W05, and W06 were very similar (V bur = 29.6,28.5, and 28.9%, respectively).Buried volumes for the NHCs in complexes W04 -W06 were calculated based on their crystal structures [13] with the web tool SambVca 2.1 developed by Cavallo et al. [34] For the settings, recommended defaults of a 3.5 Å sphere around the metal center, bond radii scaled by 1.17, and a mesh spacing value of 0.1 Å were used; H atoms were omitted.For a representative illustration, please refer to Figures S24 and S25 (Supporting Information).As found earlier for cationic Mo imido alkylidene NHC complexes, [31] we again found no significant influence of the nature of the anionic ligand on transisoselectivity.
Only the ROMP with complexes W01, W03, and W06 yields poly(M1) with low polydispersity indices (Ð ≤ 1.4) and molecular weights close to the theoretical one, suggesting favorable initiation kinetics and controlled, yet not perfectly living polymerizations (Table 1).Initiators W02 and W05 produce bimodal distributions while initiators W07 -W09 bearing the O-2,6-Ph 2 -C 6 H 3 ligand produce polymers with 3.0 < Ð < 3.6.These unusually high Ð-values may be attributed to both, low initiation efficiencies and slow polymerization kinetics, caused by the sterically demanding O-2,6-Ph 2 -C 6 H 3 ligand, which is in line with the low conversions and high M n -values found in polymerizations catalyzed by W08 and W09.Poly(M1) prepared by any of the initiators was amorphous; the glass transition temperatures were in the range between 55.9 < T g < 87.8°C.Poly(M1) with a high trans-it base generally possessed higher T g values than those with a high cis-st base (Table 1); however, a strict correlation of T g with the trans-it content of the polymers was in view of the different molecular weights of the polymers not possible.

Stereoselective ROMP: Mo-Imido versus W-Imido/Oxo/Sulfide Alkylidene NHC Complexes
The proposed reaction sequence that accounts for the formation of trans-it structures is outlined in Scheme 1 (top). [7,8,11,32,35] is based on the finding that we could not observe any signals for the anti-isomers in the starting complexes W01 -W10, which is, however, no proof for their non-existence.In the formation of trans-it polymers, a monomer adds ene anti , i.e., with the C7-carbon of the NBE down, to a syn-isomer of the initiator.The intermediary tungstacyclobutane must be stable enough to undergo turnstile rearrangement at tungsten, which consists of several Berrytype rotations.Cycloreversion then leads to a syn-first insertion product and, after multiple monomer insertions, to a trans-it polymer.With Mo imido alkylidene NHC complexes, this turnstile rearrangement is favored by, e. g. large imido-ligands. [36]With tungsten oxo/sulfido complexes this is favored by a square pyramidal (SP) transition state in which the NHC is, unlike in Mocomplexes with a trigonal bipyramidal (TBP) transition state, not perfectly trans to the tungstacycoclobutane. Indeed, in a SP transition state the tungstacyclobutane can be expected to have sufficient longevity to "survive" the necessary Berry-type rearrangements.
Generally, cationic Mo imido alkylidene NHC complexes allow for higher stereospecifities in the ROMP of NBE-based monomers for which several factors can be made accountable.In cationic nitrile-free, tetrahedral (t d ) complexes, Mo adopts a trigonal bipyramidal (TBP) transition state with the molybdacyclobutane trans to the strongest -donor, [37][38][39], i.e., the NHC, which allows the steric influence of large anionic ligands such as (substituted) terphenolates that are trans to the imido ligand to become most effective, thereby favoring the formation of cis-polymers.In the corresponding cationic W imido/oxo/thio alkylidene NHC complexes, the transition state may alternatively adopt a SP geometry (vide supra), which substantially reduces the steric pressure of large anionic ligands and, consequently, stereospecifity.Previous studies also showed that cationic pentacoordinated Mo imido alkylidene NHC complexes bearing an additional acetonitrile can react with a substrate in either an associative or dissociative manner. [19]Similar can be expected for the analogous cationic W imido/oxo/thio alkylidene NHC complexes.And indeed, the 1 H NMR spectrum of the reaction of two equivalents M1 with W06, which was chosen since it produces almost equal amounts of trans-it and cis-st polymers, reveals some interesting aspects.Thus, the spectrum shows several new Scheme 1. Formation of trans-it and cis-st structures with cationic tungsten imido alkylidene NHC complexes.alkylidene signals at  = 10.42,10.39, 10.31, 9.85, and 9.83 ppm, all doublets with 3 J-coupling constants between 3.3 and 4.1 Hz, assignable to the 3 J-coupling of the alkylidene proton to the tertiary hydrogen in the cyclopentane unit.Unfortunately, the 1 J CH coupling constants of the alkylidene protons, which would allow for an unambiguous assignment of syn-and anti-rotamers, could not be determined.However, based on literature data, [14,23] we tentatively assign the signals at  = 9.85 and 9.83 ppm to the nitrile-free syn-isomers and those at  = 10.42,10.39, and 10.31 ppm to the nitrile-containing syn-isomers.In the olefinic region, several sets of pseudo-triplets and doublets are observed at  = 5.48 (d, 3 J = 11.4Hz), 5.47 (d, 3 J = 12.2 Hz), 5.12 (t, 3 J = 11.4Hz), 4.94 (t, 3 J = 11.4Hz), 4.88 (t, 3 J = 11.3Hz), 4.87 (t, 3 J = 11.2Hz) and 4.81 (t, 3 J = 11.2Hz), all assignable to cis-configured -(cyclo-C 5 H 8 )-CH = CH-R double bonds in the short chain oligomers.In addition, doublets of doublets can be identified at  = 5.23 ppm (dd, 3 J 1 = 15.7 Hz, 3 J 2 = 7.5 Hz), 5.20 ppm (dd, 3 J 1 = 15.9Hz, 3 J 2 = 7.0 Hz) and at  = 5.04 ppm (dd, 3 J 1 = 16.3Hz, 3 J 2 = 6.7 Hz), assignable to trans-configured -(cyclo-C 5 H 8 )-CH = CH-R double bonds.According to 1 H-1 H correlated spectroscopy (Figures S11 and S12, Supporting Information), the corresponding signals (doublets) for the trans-configured -(cyclo-C 5 H 8 )-CH = CH-CMe 3 olefinic proton were in the range of 5.4-5.5 ppm but could not be identified unambiguously.
The occurrence of both, cis-and trans-configured double bonds together with the alkylidene signals for both, nitrile-containing and nitrile-free propagating species suggests the existence of two polymerization mechanisms being operative at the same time.The first involves a nitrile-free syn-configured initiator that reacts with the monomer in an ene anti fashion to yield a syn-insertion product with a trans-configured double bond via a turnstile mechanism (syn-(n-1) trans , Scheme 1, vide supra).The second entails a pentacoordinated nitrile-containing syn-isomer that inserts the monomer in an ene syn fashion to yield a syn-insertion product with a cis-configured double bond (syn-(n-1) cis , Scheme 1) In the latter case, the transition state is hexacoordinated and cannot undergo a turnstile arrangement that is only observed in pentacoordinated complexes.Since either the nitrile or the anionic ligand is in an apical position in the transition state, an ene anti approach seems less favored for steric reasons while this approach is possible in the solvent-free t d complexes.Since the chloro-complexes W01 -W03 show similar stereoselectivity than W07 -W09 bearing a large anionic ligand we propose the nitrile to be in the apical position in the transition state.In view of the observed stereospecifity of complexes W01 -W10 and of published data on other cationic W imido/oxo alkylidene NHC complexes, we propose that nitrile dissociation from cationic W imido/oxo/thio alkylidene NHC complexes in the presence of substrate is on the same time scale than coordination of substrate, which leads to both, nitrile-containing and nitrile-free transitions states.Indeed, this is different from what is observed at least for some cationic Mo imido alkylidene NHC complexes. [14]Notably, fast solvent dissociation has also been proposed by Schrock et al. for neutral Mo imido alkylidene complexes. [40,41]The two different transition states proposed here (penta-vs.hexacoordinated) might also be reason for the different regioselectivity (-vs.-insertion) observed in the cyclopolymerization of ,-diynes with solvent freeand solvent containing catalysts. [42,43]

Conclusions
Nitrile-containing cationic tungsten sulfido alkylidene Nheterocyclic carbene (NHC) complexes allow for the stereoselective ROMP of endo, exo-2,3-dicarbomethoxy)norborn-5-ene ((+)-DCMNBE).Trans-isospecifities up to 84 % can be obtained and are comparable to those obtained with most other (nitrile-containing) cationic tungsten imido and tungsten oxo alkylidene NHC complexes.The lower isospecifity compared to nitrilecontaining and nitrile-free cationic Mo imido alkylidene NHC complexes is attributed to both, a square-pyramidal instead of a trigonal bipyramidal transition state and the action of a nitrilefree and a nitrile-containing initiator, respectively, which results in penta-and hexacoordinated transition states.Implications of these findings on the regioselectivity in the cyclopolymerization of ,-diynes are currently under investigation.

Experimental Section
General: All reactions were performed in the absence of moisture and air using standard Schlenk techniques unless indicated otherwise.Reactions with metal complexes were performed in a glove box filled with nitrogen (MBraun Labmaster 130).Glassware was stored overnight at 120 °C and cooled in an evacuated antechamber.CH 2 Cl 2 , diethyl ether, toluene, pentane, and THF were dried using an MBraun SPS-800 solvent purification system with alumina drying columns and stored over 4 Å Lindetype molecular sieves.Anhydrous benzene was purchased from Sigma-Aldrich; acetonitrile was purchased from Sigma-Aldrich, dried over CaH 2 , destilled and stored over 4 Å Linde-type molecular sieves inside a nitrogen filled glove box.Deuterated solvents (Eurisotop) were used as purchased and stored over Linde-type 4 Å molecular sieves in a glove box.Initiators W01 -W10 as well as monomer M1 were prepared according to the literature. [11,12,31] 1H and 13 C NMR spectra were recorded using a Bruker Avance III 400 spectrometer at 400 and 100 MHz, respectively.COSY spectra were recorded on a Bruker Ascent 700 MHz spectrometer.Chemical shifts were given in ppm of tetramethylsilane, with solvent resonance from the remaining solvent protons (CDCl 3 : 7.26 ppm, CD 2 Cl 2 5.32 ppm) for reference.Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, quint = quintet, sept = septet, br = broad, m = multiplet), coupling constant (Hz), and integral.IR spectra were measured on a Nicolet alpha spectrometer.Size exclusion chromatography (SEC) was performed in CHCl 3 .The system consisted of a 1260 Infinity system (Agilent Technologies Inc.) equipped with a precolumn (8 × 50 mm) and three consecutive separation columns (8 × 300 mm, PSS, Mainz, Germany, porosities 1000, 100 000, and 1000 000 Å, particle size 5 μm) and an Agilent 1200 Series G1362A RI detector.The flow rate was set to 1.0 mL min −1 ; the column oven temperature was set to 35 °C.An injection volume of 100 μL was used.The system was calibrated with narrow polystyrene standards (800 ≤ M n ≤ 3000 000 g mol −1 ).DSC measurements were carried out on a Perkin Elmer DSC 4000 with a Perkin Elmer Intracooler 2p Cryostate.Data were analyzed with the Pyris software.
Typical Polymerization Procedure: To 0.5 mL of a stock solution of M1 (500 mm) in CDCl 3 0.5 mL of a stock solution of the catalyst (5 mm) was added and the reaction was stirred for 24 h at room temperature.The polymers were precipitated in methanol and dried under reduced pressure.
Investigation of the First Insertion Products: A solution of the initiator (17.8 mg, 0.01 mmol) was dissolved in CDCl 3 (0.685 mL), dodecane was added as internal standard and the solution was subjected to 1 H NMR.Then, M1 (4.18 mg, 0.02 mmol) dissolved in CDCl 3 (0.015 mL) was added and the resulting solution was subjected to 1 H NMR again.